In manufacturing electronic devices, such as semiconductor devices and liquid crystal display devices, in a photolithography process, a projection exposure apparatus is used which projects a pattern image of a mask or reticle (hereinafter called “reticle”) having a pattern formed thereon on each projection (shot) area on a substrate which is coated with a photosensitive agent (resist) via a projection optical system. Circuits of an electronic device are transferred by exposing a circuit pattern on a substrate to be exposed by the projection exposure apparatus, and formed in a post-process.
Recently, large scale integration of integrated circuits or miniaturization of circuit patterns are in progress. In this respect, the wavelength of illumination light for exposure (hereinafter called “exposure light”) to be used in the projection exposure apparatus tend to become shorter. That is, a light source of a short wavelength, such as a KrF excimer laser (wavelength: 248 nm) is taking the place of a mercury lamp that has been popular, and an ArF excimer laser (193 nm) is coming to the last stage of the practical use. Exposure apparatuss that use an F2 laser (157 nm) are being developed to achieve higher integration.
Beams having a wavelength of about 190 nm or shorter belong to a vacuum ultraviolet region and do not pass through the air. This is because a substance, such as oxygen molecules, hydrogen molecules or carbon dioxide molecules, present in the air (hereinafter called “light absorptive substance”) absorbs the energy of the beam.
In an exposure apparatus using exposure light of the vacuum ultraviolet region, it is necessary to reduce or eliminate the light absorptive substance from the space of the optical path of the exposure light for the exposure light to reach a substrate to be exposed with sufficient illumination. For this purpose, it is often the case where in the exposure apparatus, the space above the optical path is surrounded by a casing and the space inside the casing is filled with a permeable gas which passes exposure light. In this case, given that the total optical path length is 1000 mm, for example, the density of the light absorptive substance in the space of the optical path is practically is equal to or less than 1 ppm or so.
As a substrate is frequently replaced in the exposure apparatus, however, it is difficult to eliminate the light absorptive substance from the space between the projection optical system and the substrate in the space of the optical path. For instance, while it is preferable to set a casing large enough to surround even a mechanism for substrate replacement, the consumption amount of gas to be filled in the casing becomes larger as the casing becomes larger in this case.
In this respect, there is a case where an exposure apparatus uses a technique of removing a light absorptive substance from the space of the optical path by blowing a permeable gas, which passes exposure light, on the exit portion of the projection optical system. This technique is described in, for example, Japanese Patent Laid-Open Publication No. H6-260385.
As the blown gas is likely to leak around a substrate according to the technique, however, the leaked permeable gas may affect peripheral system. For example, an exposure apparatus often employs an interferometer system using a laser beam in order to control a stage where a substrate is to be mounted. When a permeable gas enters the optical path of the interferometer, however, the difference between refractive indexes of a gas (air) which has been present on the interferometer optical path so far and the light absorptive substance changes the optical path length of the laser beam, which may lower the control precision of the interferometer system.
When a substrate is placed on the exit portion of the projection optical system, the substrate becomes a part of the partition to suppress the gas leakage, but when the substrate moves or the substrate is replaced, at least a part of the substrate which becomes the partition comes off the exit portion of the projection optical system, so that the gas state on the exit portion of the projection optical system is not kept and the gas leakage is likely to occur.
The invention has been made in consideration of the above-described situations, and aims at providing an exposure method and an exposure apparatus which can adequately remove a light absorptive substance from the exit portion of the projection optical system and can keep the gas state even at the time of moving or replacing a substrate.
It is another object of the invention to provide a device manufacturing method which can improve the pattern precision.